1. Field of the Invention
The present invention relates generally to a float switch device for use in a liquid medium, and more particularly to a float switch having a magnetic latching feature used to produce an offset, with the switch actuating in a first manner when the liquid level drops to a first level, and actuating in a second manner when the liquid level rises to a second level which is above the first level by a selected offset distance.
2. Description of Related Art
Liquid level sensing float switches have been in use for some time to control the execution of an operation used to maintain liquid level in a reservoir at a desired level or within desired limits. The basic float switch has a float element which is free to move vertically on a support element, with the float controlling the actuation of a switch element as it moves up and down with respect to the support element. When liquid level falls below a set point, the float switch will actuate in a first manner, and when the liquid level rises above the set point the float switch will actuate in a second manner.
Offset may be defined as the distance between the level at which the float switch actuates in the first manner and the level at which the float switch actuates in the second manner. In a typical float switch, there may be very little offset, making the switch highly vulnerable to any kind of turbulence which may occur in the reservoir. It will be appreciated by those skilled in the art that there exist a wide variety of float switches, which are designed to serve more than one function.
For example, a float switch may be used to provide an indication of liquid level within a reservoir. Examples of such float switches are found in U.S. Pat. No. 3,242,474, to Gast et al., in U.S. Pat. No. 3,997,744, to Higo, in U.S. Pat. No. 4,175,435, to Hara, and in U.S. Pat. No. 4,442,405, to Andrejasich et al. The Higo device uses a permanent magnet mounted on a float to operate a magnetic reed switch, and the Andrejasich et al. device has a float which moves close to a fixed permanent magnet to operate a magnetic reed switch. As such, both of these devices will have virtually no offset, since a magnetic reed switch has perhaps one-sixteenth of an inch offset.
The Hara device, like the Higs reference, uses a permanent magnet mounted on a float to operate a magnetic reed switch. In one embodiment, the Hara reference has four such switches located in a vertical array to indicate various liquid levels in the reservoir. The Gast et al. device uses a permanent magnet in the float to repel a second permanent magnet on the switch, thereby closing the switch. While Gast et al. states that this in theory will reduce the susceptibility of the device to turbulence, in practice the Gast et al. device is not really superior to any of the other switches described above.
Other references in addition to the alternative embodiment of Hara cite the use of multiple float mechanisms to indicate multiple levels. Specifically, such references include U.S. Pat. No. 438,598, to Ashton, U.S. Pat. No. 2,536,273, to Gahagan, U.S. Pat. No. 2,736,013, to Binford, and U.S. Pat. No. 3,483,342, to Mauro. The Ashton device has two float switches, one of which alarms if the liquid level is too high, and the other of which alarms if the liquid level is too low. The Gahagan device also has two float switches, one of which is used to monitor liquid level, and the other of which is a density responsive switch used to detect the presence of water in a lubricant reservoir.
The Binford reference teaches a device having two interconnected float switches to provide indications of low and high liquid level, respectively. The Mauro reference likewise uses a plurality of float switches, with low and high switches being used in an oil reservoir, and a low water switch being used in a water reservoir. All of these devices have only a very small amount of offset, and they are designed for use primarily as level indicators.
Another desirable use of a float switch is to control the operation of a pump which drains liquid from a reservoir, as in the case of a sump pump. When the liquid level in the reservoir falls below a low set point, the switch turns the pump off. When the liquid level in the reservoir rises above a high set point, the switch turns the pump back on to pump liquid from the reservoir. Conversely, operation of a pump may also be controlled by a float switch to pump liquid into a reservoir to maintain liquid level at a desired level. In either event, in order for the system to function properly, there must be a sufficient degree of offset between the low set point and the high set point.
If there is not a sufficient degree of offset, several problems arise in the system. First, it will be apparent to those skilled in the art that if there is any turbulence whatsoever in the reservoir, systems having only a minimal offset will repeatedly cycle on and off, an undesirable effect. Secondly, if there is only a small offset in liquid level between switch cycling, the pump will be run very frequently.
For example, consider a sump pump or a similar system. The pump will shut off when the low set point is reached. As soon as the high set point is reached, the pump will be turned on again, with little time between pumping cycles and very short pumping cycles. It is apparent that such a system will be very hard on the pump, reducing its operating life. Even more seriously, if there is only a small offset distance, the pump could cycle on, pump enough liquid to cycle off, and have sufficient liquid backflow into the reservoir to cycle the pump on again. This situation would result in a continuously cycling pump, an undesirable effect.
A system may overcome these problems by using two float switches and a relay system to cycle the pump on and off. In this case, one of the float switches is set for the high set point, and the other is set for the low set point. The relay system cycles the pump on, for example, at the high set point, and off at the low set point.
Alternatively, a system may use a mechanical system to perform the same function. One such reference is found in U.K. Patent Application 2,047,468A, to Weston. A magnetic reed valve is used, with the small offset being mechanically multiplied by the design of the system. Two floats are used, with the mechanical tolerances being critical and requiring precise adjustment. In addition, since a magnetic reed switch is still used, the system will remain highly susceptible to turbulence in the liquid in the reservoir. As a result, the actuation of the Weston device is not crisp and positive.
Two other references of interest are shown in U.S. Pat. No. 2,915,605, and in U.S. Pat. No. 2,999,913, both to Friedell, and both illustrating the same two devices. The first device illustrated by the Friedell references uses a single float with magnetic attraction between a permanent magnetic collar in the float and a permanent magnet in the supporting shaft. The device includes a double pole, double throw switch, and cycles between closing one set of contacts and the other. The device is free from the problem of cycling due to turbulence, but it offers only a limited amount of offset. In addition, it is mechanically complex, having over twenty parts. As such, it is likely to be expensive and difficult to manufacture.
The other device illustrated in the two Friedell references uses two floats, with the lower float having the same permanent magnetic collar/permanent magnet arrangement as described above. When the liquid level is sufficiently high to float the upper float, near the top of the reservoir, a rod connected to the upper float will lift the lower float, freeing the magnetic coupling between the lower float and its support. Otherwise, the upper and lower floats are not connected together. This system is also mechanically complex, with even more parts than the other Friedell system described above. As such, it will also be mechanically complex and expensive to manufacture.
It is accordingly the primary objective of the present invention that it provide a float switch which may be used to control the operation of a pump, and that it provide a substantial degree of offset in operation between the high and low set points. It is desirable that in addition to being able to provide a substantial amount of offset, it be susceptible to manufacture as a device having any desired amount of offset. It is a further objective that the float switch of the present invention be constructed to operate only a single switch element, thus eliminating the requirement for a control system such as relays.
The operation of the float switch of the present invention in cycling from off to on and from on to off must be both crisp and positive, with the points at which it will switch being absolutely certain. The float switch should be able to operate in a reservoir where it may experience a degree of turbulence, with the turbulence not substantially affecting the operation of the device. In addition, it must be absolutely free from undesirable oscillations between the on and off states caused by turbulence or small changes in liquid level in the reservoir.
It is also an objective that the float switch of the present invention be as mechanically simple as possible, having a single moving float element. It should have as few parts as possible, both moving and fixed, to make its construction simple and its operation dependable and long-lasting. It should also be of inexpensive construction, thereby giving it an economic advantage over more mechanically complex devices. It is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.